1. Field of the Invention
The present invention relates to an image capturing apparatus having an automatic focusing function.
Priority is claimed on Japanese Patent Application No. 2005-294509, filed Oct. 7, 2005, the content of which is incorporated herein by reference.
2. Description of the Related Art
In image input apparatuses such as video cameras or digital cameras, a zoom function is widely used for freely expanding or contracting the image in accordance with the distance to a subject to be photographed, or the size of the subject in the angular field, whereby it is executed by changing the focal length of the lens. The zoom function is classified into: (i) optical zooming generally implemented by mechanically moving internal lenses, and (ii) electronic zooming for performing image expansion by using a part of the image output from the image sensor and interpolating new pixels between existing pixels so as to generate an interpolated image. In comparison with the optical zooming, the electronic zooming includes no elements to be driven, and can be realized as a small device at low cost; however, image quality thereof is inferior.
With respect to such a problem, Japanese Unexamined Patent Application, First Publication No. H10-233950 discloses an electronic zoom image input system (see FIG. 12) having: an optical system for optically compressing a periphery of an input image; and a light receiving element, having a substantially uniform pixel density, for photo-detecting the input image. This electronic zoom image input system has a function of correcting and converting the image which is detected by the light receiving element and includes distortion due to the above compression, thereby producing a zoom image having a resolution equivalent to the optical zooming.
FIG. 12 shows a telephoto angular-field image plane 1AS, a middle angular-field image plane 1AM, and a wide-area angular-field image plane 1AW on a subject plane 1A. These planes 1AS, 1AM, and 1AW respectively correspond to image planes 1BS, 1BM, and 1BW (with respect to compression) formed on the photodetective plane of an imaging device. Image data captured through the photodetective plane is appropriately corrected, so that a telephoto angular-field image plane 1CS, a middle angular-field image plane 1CM, and a wide-area angular-field image plane 1CW are each captured as a corrected zoom image plane 1C (i.e., a converted normal image). Accordingly, electronic zooming can be performed without degrading general resolution while the resolution varies locally.
On the other hand, Japanese Unexamined Patent Application, First Publication No. 2001-255450 discloses an automatic focusing method used in an image input apparatus such as a video camera or a digital camera. FIG. 13 shows a block diagram relating to the disclosed method.
In FIG. 13, light incident via a focusing lens 100 is subjected to photoelectric conversion in a CCD element 101, thereby generating image signals which are each converted to a digital signal by an A/D converter. This digital signal is input into a signal processing circuit 103 in which the signal is subjected to correction processes such as white balance correction, γ correction, and the like. The processed signals are sequentially stored in a DRAM 104.
In a processing circuit 105, each image signal read out from the DRAM 104 is subjected to a signal processing such as color separation. Each image signal processed by the processing circuit 105 is stored in a storage medium 106.
Each image signal stored in the DRAM 104 is also supplied to a gate circuit 107. Based on a command issued from a CPU 112, the gate circuit 107 assigns each image signal to one of a plurality of independent areas, so as to set focusing areas (in FIG. 13, a control line between the gate circuit 107 and the CPU 112 is not shown).
Each image signal assigned to any one of the independent areas by the gate circuit 107 is converted to a brightness signal by a brightness signal generating circuit 108, which is then supplied to a HPF (high-pass filter) 109 and a LPF (low-pass filter) 110. The HPF 109 extracts a high-frequency signal component from each brightness signal, and sends the extracted signal component to an integrator 111. The integrator 111 integrates the high-frequency signal component, and sends the result of the integral to the CPU 112 as a focus evaluation value. The LPF 119 extracts a low-frequency signal component from each brightness signal, and sends the extracted signal component to the integrator 111. The integrator 111 integrates the low-frequency signal component, and sends the result of the integral to the CPU 112 as brightness data. Based on each focus evaluation value obtained by the integrator 111, the CPU 112 drives a motor 113 for changing the position of the focusing lens 100, so as to perform focusing.
As the method of driving the motor 113 for focusing, a so-called “hill-climbing control method” is used in the above example. Each focusing area has a position at which the focus evaluation value is maximum. The CPU 112 determines that the nearest position (to the camera) of such positions of the focusing areas is a focal position to be set, and drives the motor 113 so as to move the focusing lens to the focal position. Such control is performed because generally in camera photography (in particular, when taking portraits), a desired subject to be photographed is present near to the camera, while distant objects belong in the background. As discussed above, the focal position is determined by monitoring the variations in the focus evaluation values computed with respect to the plurality of focusing areas; thus, focusing can be appropriately performed for various types of subjects.
In each image obtained by the image input apparatus disclosed in the above publication H10-233950, a central part and a peripheral part have different characteristics. That is, the central part, in which the image is expanded, includes a large number of low-frequency signal components, while the peripheral part, in which the image is compressed, includes a large number of high-frequency signal components. In camera photography (in particular, in order to take portraits), a desired subject is generally positioned near to the camera and in the central part of the image, as discussed above.